Electrolytic process for producing phosphine



April 4, 1967 G. T. MILLER 3,312,610

ELECTROLYTIC PROCESS FOR PRODUCING PHOSPHINE Filed March 4 TO STORAGEONDENSER United States Patent 3,312,610 ELECTROLYTIC PROCESS FORPRODUCING PHGSPHINE George T. Miller, Lewiston, N.Y., assignorto HookerChemical Corporation, Niagara Falls, N.Y., a corporation of New YorkFiled Mar. 4, 1963, Ser. No. 262,496 8 Claims. (Cl. 204-101) Thisinvention relates to a process for the preparation of phosphine. Moreparticularly, it relates to electrolytic processes for the preparationof phosphine, including suitable apparatus for elfecting such processes.

In preparing phosphine electrolytically, molten phosphorus reacts withhydrogen gas or ions being produced at one of the electrodes in the cellto yield phosphine, one of the desired products. Other end products arealso obtainable, depending on the electrolyte utilized in the cell.During the usual operation of such cells, it has been observed that thephosphorus is not consumed completely and becomes viscous after a periodof time. When this occurs, it is the phosphorus that is replenished orreplaced because the viscous phosphorus reduces the efiiciency of thecell and decreases the yield of phosphine. The proc ess and apparatus ofthe present invention provides for an efficient utilization ofphosphorus and greatly increases the yield of phosphine therefrom.

It has been found by the present invention that in the electrolyticcells herein described that phosphorus will ;remain in a non-viscousstate when it is purged'with a reducing gas. In accordance with thisinvention, phosphine may be prepared by contacting an anode and acathode with an electrolyte, a portion of said cathode being in contactwith phosphorus, maintaining a layer of phosphorus on a surface of saidcathode, maintaining an electric current between said anode and saidcathode through said electrolyte, and purging said phosphorus with areducing gas, so that the phosphorus is maintained in a non-viscous formand phosphine is recovered in the region of the cathode.

It has been also found that when the reducing gas is passed through thephosphorus of an electrolytic phosphine cell, the phosphoruscontainedtherein more readily proceeds up and remains in contact with thecathode, to maintain a thin layer of phosphorus on the surfaces of thecathode. It has been also found that the required current density in thecell is increased, thereby allowing a decrease in electrical powerconsumption, producing a high yield of phosphine.

An electrolytic cell that has been found successful in carrying out theoperation of this invention comprises an anode and cathode, anelectrolyte in operative relationship with the anode and cathode, liquidphosphorus in contact with said cathode so that by passing an electriccurrent through the electrolyte phosphine is generated in the catholytegas, and means for passing a gas through the phosphorus to maintain itin a non-viscous condition. Of course, there must be a source ofhydrogen for phosphine production and this may be an aqueous electrolyteor one otherwise capable of yielding hydrogen to react with thephosphorus.

The various aspects of the invention will be apparent from the followingdescription, taken with the drawing illustrative of an embodiment of theinvention, in which FIG. 1, is a central vertical sectional view of theapparatus of this invention along 1-1, and FIG. 2 is a horizontalsectional view along 2-2.

Referring to FIG. 1, cell vessel contains anode compartment 12, anode14, cathode compartment 16, and cathode 18. A porous or permeablediaphragm separates anode and cathode compartments and separates thePatented Apr. 4, 1967 electrolyte into anolyte 17 and catholyte 19sections. Phosphorus 24 in a liquid state is present in the cathodecompartment 16. Diaphragm 20 is covered or coated on the side thereoffacing the liquid phosphorus, where it may otherwise contact thephosphorus, with a coating, cover or sheath 22, against which phosphorusdoes not adhere, so that it does not wet the diaphragm, which action isevidenced by convex meniscus 21. Ports 26 and 28 permit the addition andremoval, either continuously or batchwise, of anolyte from anode section12. Ports 30 and 32 permit the addition and removal, either continuouslyor batchwise, of catholyte from the cathode section 16. Port 34 permitsthe addition and removal of phosphorus from a cathode section 16.Sufiicient phosphorus 24 is added to the cathode compartment 16 tocontact the lower portion or edge of cathode 18, permitting contact ofthe phosphorus with a greater surface of the cathode by a wettingaction. Anolyte gas discharge port 36 is provided in the top of theanode section 12 to remove anolyte gas from the electrolytic cell.Catholyte gas discharge port 38 is provided on the top of cathodesection 16 to remove catholyte gas. Gas-liquid interfaces are indicatedat 15. Conduit 60 carries the catholytic discharge gas to gas-liquidseparator means 62, which may be a condenser, centrifuge or othersuitable apparatus. Here the gas and liquid electrolyte, which may becarried over, are separated. The liquid phase 19 is returned to theelectrolyte 19 in the cell by conventional return means or pipe 61.Conduit 64 carries the catholyte gas to condenser 66, where some of theelectrolyte which has come over in a gas phase may be condensed andreturned to the cell 10. Uncondensed gases principally phosphine arethen delivered through conduit 68 to pump 70 or to a storage area orother suitable receiver. Generally, only a minor proportion of thecatholyte gas is shunted through conduit 68 to be recycled throughconduit 72 via pump 70 through port 73 and into the phosphorus 24 at 74.

Electrolyzing current to the electrodes is transmitted by anodeelectrical cable or connector 46 and cathode electrical cable orconnector 48, joining the anode and cathode to the positive and negativepoles at plugs 42 and 43, respectively, of a source of direct current50.

If desired, a heating or cooling means such as a constant temperaturebath, not shown, may be employed to maintain the cell at or near adesired operating temperature.

Cell vessel 10 may be constructed of any material capable of resistingcorrosion by the electrolyte and other materials employed in the cell.Typical examples of suitable materials of construction for cell vessel10 include glass, glazed ceramics, tantalum, titanium, hard rubber,polyethylene, rigid materials coated with phenol-formaldehyde resin andthe like.

The purging gas of this invention may be any suitable reducing type gas.Suitable reducing gases include hydrogen, phosphine, hydrogen sulfide,carbon monoxide, hydrogen cyanide, saturated and unsaturated hydrocarbongases, such as ethane, methane, butane, butene, acetylene gases, andother natural fuel type gases and mixtures thereof. The gases herein maybe delivered from an external or internal source through conduit 72,purge the phosphorus, and leave through port 38. It is highly preferredthat a portion of the cathodic gas is recycled to purge the phosphorus.The rate at which this gas is delivered to the phosphorus may vary frombetween about 0.5 liter and liters per hour per square foot of cathodearea. However, between about 0.5 liter and 25 liters per hour per squarefoot of cathode are preferred. However, best results are achieved usingbetween about 1 and 20 liters per hour per square foot of cathode area.Of course other proportions may be used, but the operation in such casesusually will not be as successul. The pro- 3 portion of cathode gaswhich may be recycled may vary between about and 90 percent of the totalgas produced at the cathode. The preferredamount of cathode gas whichmay be recycled is between about 60 and 75 percent of the total cathodegas. However, optimum results are obtained when between about andpercent of the cathode gas is recycled. The purging of phosphorus with areducing gas preferably by recycling of the oatholyte gases, results ina manifold benefit for the production of phosphine. An increased currentdensity results at the cathode, the phosphorus utilized in theproduction of phosphine in the cell may be maintained within the cellfor a longer period of time, thus yielding more phosphine per unit ofphosphorus placed in the cell, and phosphorus is made to contact thecathode more efficiently due to the formation of phosphorus droplets asfilms which adhere and climb or wick-up the cathode surface morereadily.

Although the exact mechanism of the action of the gas is not known, itis believed that the gas utilized to purge phosphorus should be onewhich prevents phosphorus hydrates from forming. However, this inventionis not intended to be limited by this disclosed possible mechanism.

The viscosity of elemental phosphorus is about 1.55 centipoises measuredat about 50 degrees centigrade. When the viscosity of the phosphorus inthe cell is about 8 or 9 centipoises at 50 degrees centigrade, the cellefficiency is reduced to such a level that the phosphorus should bereplenished for efiicient operation. 'It has been found that by blowing,purging, agitating, circulating, bubbling, or reducing the pool ofphosphorus with a reducing gas, the viscosity is usually maintainedbetween 1.6 and 4.5 centipoises. Very favorable results also areobtainable when the viscosity is maintained at between about 1.55 and 7centipoises by a reducing gas, all measurements being taken at 50degrees centigrade on an Ostwalk-Fenshi type viscometer.

Diaphragm 20 which separates the anode section 12 from cathode section16, may be a porous or semipermeable material resistant to the cellcontents and capable of maintaining the anode and cathode gasesseparate, but allowing the electrolyte to pass through. The phosphorusis also held in the catholyte section of the cell. Typical examples ofsuitable materials for use as a diaphragm include porous Alundum, porousporcelain, resin impregnated wool felt, and various other separators ofthe types which may be normally employed in the lead storage batteries.

Solid materials having a hydrogen overvoltage, as normally measured inthe absence of phosphorus, exceeding the hydrogen overvoltage of smoothplatinum may be desirably employed as the cathode. Typical cathodicmaterials include lead, amalgamated lead, cadmium, tin,

aluminum, nickel, alloys of nickel, such as Mu Metal (an alloycontaining 77.2 percent nickel, 4.8 percent cop- 7 per, 1.5 percentchromium, and 14.9 percent iron), Monel, copper, silver, bismuth, andalloys thereof. For example, lead-tin, lead-bismuth, and tin-bismuthalloys may be employed. Various shapes of cathodes may be employed. Forexample, the cathode may be a plate as illustrated in the drawing. Matsof metallic wool and porous metal sheets may also be employed ifdesired. It is also suitable to utilize a liquid molten mercury cathodein the practice of the present invention.

Suitable anode materials include lead, platinum, lead peroxide, graphiteand other materials of construction capable of conducting current andresisting corrosion under the conditions of electrolysis employed.

In one embodiment of the invention, a lead plate with grooves isemployed as the cathode and a graphite plate is employed as the anode.Under these conditions, it has been found that the wicking effect of thelead cathode is markedly improved.

The electrolyte may be a salt or other organic or inorganic electrolytewhich is non-reactive with molten phosphorus and which is capable offorming hydrogen gas or ions under the electrolytic conditions employed.Typical examples of suitable compounds in aqueous solution, which may beemployed as the catholyte include hydrochloric acid, sodium chloride,lithium chloride, potassium chloride, sodium sulfate, potassium sulfate,monosodiumphosphate, disodiumphosphate, acidic acid, ammonium hydroxide,phosphoric acid, sulfuric acid, and mixtures thereof. The concentrationof a compound in the aqueous electrolyte may vary from about 1 to about95 percent, but is usually between about 5 and about percent, and ispreferably between about 10 and about 50 percent. Suitable non-aqueouselectrolytes may also be employed in the present invention, e. g.,sodium hydride.

Improved results have been obtained when metallic ions are present inthe electrolyte in small proportions.

Suitable concentrations of the metal ions may be between about 0.01 and5 percent by weight of the electrolyte. However, between about 0.02percent and 3 percent by weight of electrolyte may also be utilized.Preferably though, between about 0.02 and 15 percent by weight ofelectrolyte may be utilized. For example, ions of metals such asantimony bismuth, lead, tin, cadmium, mercury, silver, zinc, cobalt,calcium, barium, and mixtures thereof may be employed. The metal ionsmay be placed in the electrolyte by employing a consumable anode of thedesired metal or metals such as a lead anode,

whereby metal ions are formed in the catholyte and transferred to thearea adjacent to the cathode. Salts or other compounds of the metalssuch as chlorides, phosphates, acetates, and the like may be dissolvedin the electrolyte if desired. In another embodiment, finely dividedmetal in elemental form is dissolved in the electrolyte.

During electrolysis, the temperature of the catholyte and anolyte shouldbe maintained above the melting point of phosphorus (about forty-fourdegrees centigrade), and below the boiling point of the electrolyte.Temperatures between about sixty degrees centigrade and one hundred andten degrees centigrade are generally satisfactory, but optimum yields ofphosphine are obtained at temperatures between about seventy degreescentigrade and about one hundred degrees centigrade. When an electriccurrent is passed through the cell, molten phosphorus on the surface ofthe cathode is consumed in the formation of the catholyte gas in thecathode section. The catholyte gas is predominantly phosphine, butcontains some hydrogen. A portion of this gas may be recycled and passedthrough the phosphorus in the cell. The anolyte gas composition dependson the overvoltages of the anions in the anolyte with reference to theanode material, as well as on the electrolyte. Thus, for example, theanolyte gas predominates in oxygen if sulfuric acid or phosphoric acidis utilized with a platinum anode, whereas for the same anode, chlorinepredominates if hydrochloric acid is used as an anolyte. Thus, thecop-reduction of anodic oxidation products may be carried out in theanode compartment of the cell of this invention without departing fromthe spirit of the invention.

The phosphine-containing gas produced at the cathode has a relativelyhigh concentration of phosphine usually more than 60 percent, and it maybe as high as percent phosphine by volume or higher. The catholyte gasis substanti-ally free from other phosphorus hydrides.

The following examples are presented to illustrate the inventionfurther, without limiting it. All parts in the specification, examplesand claims are by weight and all temperatures are in degreescentig-rade, unless otherwise specified.

Exa'mple 1 Liquid phosphorus (40 parts) was charged to an electrolyticphosphine cell. The cell contained a lead cathode and a graphite anode.The electrolyte in the cell consisted of 10 percent hydrochloric acidwith about 0.04 percent finely divided elemental lead, dissolvedtherein. The cell was divided into anolyte and catholyte sections by anAlundum diaphragm coated with glass fabric.

After addition of liquid phosphorus to the cell, direct current wasapplied to the cell at a potential of 4 volts. At the beginning of thecell operation, the cathodic gas was analyzed for phosphine gas and wasfound to be 30 to 40 percent phosphine. After seven days continuousoperation (about 326 ampere-hours) the phosphorus in the cell had becomeviscous (over 8 centipoises) and an analysis of the catholyte gas showedthat only about 8 percent phosphine was present. This operation waswithout benefit of circulation or recycling of reducing gas through thephosphorus.

Example 2 Additional melted phosphorus (60 parts) at 50 degreescentigrade was added to the phosphine producing cell of Example 1.Operating conditions were kept the same except for the use of hydrogengas to treat or purge the phosphorus. This all was operated for sevenconsecutive days with intermittent analyses of the catholyte gas. Overthis period of time, the liquid phosphorus did not become viscous andanalyses of the catohlyte gas showed between 70 and 75 percent phosphinepresent in the cathode gas.

Example 3 Phosphorus was added to the cell and Example 2 was repeatedexcept that about 40 percent of the cathode gas was recycled to purgethe phosphorus, and no extra hydrogen was used. After 24 hours, therecycling of the catholyte gas was stopped and the cell continued inoperation. The percent of phosphine in the cathode gas, without passingrecycled cathode gas into the cell, Was then analyzed over a period ofabout an hour, as indicated in Table I.

TABLE I Time in minutes: Percent phosphine present 84 5 87 86.5 17 85 2283 38 79.5 55 69 After 1 hour, about 50 percent of the cathodic gas wasagain recycled. Table II indicates the results of subse quent analyses:

TABLE II TABLE III Recycle Rate Liters Average Percent per Hour perSquare Phosphine in Foot of Cathode Cathode Gas Although the recycle ofpnoduced or external gas through the reactive material has beendescribed with respect to treating phosphorus in the electrolyticmanufacture of phosphine, it should be clear that such gas treatmentwill also be useful when other reactive materials, e.g., sulfur, are tobe kept fluid and are to be aided in wetting an electrode at which theyreact with electrolytic products. Also considered as within theinventive concept are processes in which phosphorus is reacted with anelectrolytic cell to produce materials other than PH The invention hasbeen described with respect to a preferred embodiment thereof. It is notlimited thereto, but also encompasses apparatuses and processes in whichequivalents have been substituted for the elements of the appendedclaims. a

What is claimed is:

1. A process for comprising:

preparing phosphine electrolytically (a) contacting an anode and acathode with an electrolyte, a portion of said cathode being in contactwith a body of liquid phosphorus;

(b) maintaining a layer of said phosphorus on the surface of saidcathode;

(c) maintaining an electric current between said anode and said cathodethrough said electrolyte;

(d) and passing a reducing gas into contact with and through said bodyof phosphorus so that it is maintained in liquid form and phosphine isproduced in the region of the cathode.

2. A process in accordance with claim 1 wherein the reducing gas isselected from the group consisting of hydrogen, phosphine, hydrogensulfide, carbon monoxide, hydrogen cyanide, saturated and unsaturatedhydrocarbon gases and mixtures thereof.

3. A process in accordance with claim 1 wherein the reducing gas is amixture of hydrogenand phosphine.

4. A process for preparing phosphine electrolytically comprising thesteps of:

(a) contacting an anode and a cathode with an electrolyte, a portion ofsaid cathode being in contact with a body of liquid phosphorus;

( b) maintaining a layer of said phosphorus on the surface of saidcathode; and said cathode through said electrolyte, whereby a gascomprising hydrogen and phosphine is produced at the cathode; and

(c) passing an electric current between said anode;

((1) recycling a portion of the gas produced at the cathode through thebody of phosphorus in contact with the cathode so that it is maintainedliquid.

5. A process in accordance with claim 4 wherein the gas produced at thecathode is recycled at a rate of from between about 0.5 and liters perhour per square foot of cathode.

6. A process in accordance with claim 4 wherein the phosphorus ismaintained at a viscosity between about 1.55 and 7.0 centipoisesmeasured at 50 degrees centigrade.

7. A process in accordance with claim 1 wherein the cathode is lead.

8. A process in accordance with claim 1 wherein the electrolyte iscapable of forming hydrogen gas under the electrolytic conditionsemployed.

References Cited by the Examiner UNITED STATES PATENTS 1,926,837 9/1933Cupery 204-265 2,719,822 10/ 1955 Kassel 204237 2,780,593 2/1957 Snow etal 204265 3,109,788 7/1960 Miller ct al 204-101 HOWARD S. WILLIAMS,Primary Examiner. JOHN H. MACK, Examiner. L. G. WISE, H. M. FLUOURNOY,Assistant Examiners.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,312,610 April 4 1967 George T. Miller It is certified that error appears inthe above identified patent and that said Letters Patent are herebycorrected as shown below:

Column 5, line 25, for "catohlyte" read catholyte coiumn 6, lines 44 to46, strike out "and said cathode through a1d electrolyte, whereby a gascomprising hydrogen and phosphir 18 produced at the cathode; and", andinsert the same after 'Ianode", in line 47 same column 6; same column 6,line 47, after anode" strike out the semicolon.

Signed and sealed this 22nd day of July 1969.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer

1. A PROCESS FOR PREPARING PHOSPHINE ELECTROLYTICALLY COMPRISING: (A)CONTACTING AN ANODE AND A CATHODE WITH AN ELECTROLYTE, A PORTION OF SAIDCHATHODE BEING IN CONTACT WITH A BODY OF LIQUID PHOSPHORUS; (B)MAINTAINING A LAYER OF SAID PHOSPHORUS ON THE SURFACE OF SAID CATHODE;(C) MAINTAINING AN ELECTRIC CURRENT BETWEEN SAID ANODE AND SAID CHATHODETHROUGH SID ELECTROLYTE; (D) AND PASSING A REDUCING GAS INTO CONTACTWITH AND THROUGH SAID BODY OF PHOSPHORUS SO THAT IS MAINTAINED IN LIQUIDFORM AND PHORPHINE IS PRODUCED IN THE REGION OF THE CATHODE.